Without limiting the scope of the invention, its background is described in connection with heterojunction bipolar transistors (HBTs), as an example.
Heretofore, in this field, two of the most important figures of merit in power HBT design have been the emitter-collector breakdown voltage, BV.sub.ceo, and the maximum operating current density prior to base pushout, J.sub.max. For a transistor under a given bias condition, its output power is directly proportional to the product of the operating emitter-collector voltage and the collector current density. Therefore, one would like to design a power transistor to operate at large emitter-collector voltage and collector current density. However, when the emitter-collector voltage is increased to an extreme, the base-collector junction breakdown eventually occurs and the transistor ceases to operate. The voltage at which the breakdown occurs is essentially the emitter-collector breakdown voltage, BV.sub.ceo.
On the other hand, power can also be increased by increasing the operating collector current density level. As with the case of increasing emitter-collector voltage, there is a limit to the operating collector current density level, J.sub.max, beyond which the transistor ceases to function properly. The physical effect imposing this limit of current density level is called the base pushout effect (also known as the Kirk effect). When the operating collector current density is larger then J.sub.max, the number of free carriers entering the base-collector space-charge region becomes so large that the carriers greatly modify the background charge in that region. Consequently, the electric field at the base side of the base-collector junction decreases to zero, and the base majority carriers spill over into the junction. At this point, the effective base width suddenly increases and current gain dramatically decreases, causing the transistor to cease to function properly.
In practice, both of these parameters are heavily influenced by the design of the collector, which can be varied in both thickness and doping concentration. The collector thickness of an HBT designed for high power applications is generally required to be as thick as possible, so that a large reverse base-collector junction voltage can be sustained. However, when carried to an extreme, the collector space charge transmit time becomes so large that the transistor cannot effectively function at frequencies of interest. As the thickness of the collector is increased, the time required for carriers to cross the collector (known as the collector space charge transit time, or collector transit time) also increases. When the collector transit time becomes a substantial fraction of one period or cycle at the frequency of operation, the current gain and efficiency of the transistor are drastically reduced at that frequency. With a thicker collector, a larger reverse base-collector junction voltage can be sustained and, therefore, BV.sub.ceo becomes larger. Typically, for HBTs designed for X-band operation (6.2 GHz-10.9 GHz), the collector thickness is constrained to 1 .mu.m or less. The only remaining design option is thus the collector doping. With only this one design parameter to work with, the collector doping profile must be a tradeoff between the maximization of BV.sub.ceo or J.sub.max. At one extreme, it is desired to make this collector doping level, N.sub.coll, as heavy as possible so that the base pushout effects start at a much higher current density, given as: EQU J.sub.max =q.times.N.sub.coll =V.sub.sat
where:
q is the electronic charge
V.sub.sat is the saturation velocity of the carriers
Consequently, J.sub.max is large and the transistor can be operated at higher current levels.
At the other extreme, it is desired to make the collector doping profile as light as possible, so that a higher voltage can be sustained across the base-collector junction before the junction breakdown occurs. In this manner, BV.sub.ceo can be increased, and the transistor can be operated at higher voltages.
Therefore, a problem faced with the conventional single-layer collector has been that there is virtually no design freedom for the substantially independent and simultaneous increase of both BV.sub.ceo and J.sub.max by varying just the collector doping level. Accordingly, improvements which overcome this problem are presently desirable.